Differential input and output transconductance circuit

ABSTRACT

The present invention relates to a transconductance circuit intended to convert a differential input voltage, supplied as two signals to two inputs, IN+ and IN− respectively, into a differential output current. According to the invention, each of the two signals of said differential input voltage is supplied to each input, IN+ and IN− respectively, through a follower transistor, TF+ and TF− respectively, connected to said input, IN+ and IN− respectively, by its emitter and receiving said signal on a control electrode. Moreover, each of the two inputs, IN+ and IN− respectively, of the transconductance is connected to a respective current source, CS− and CS+ respectively, that is dynamically controlled by the other input of the transconductance, IN− and IN+ respectively, said current source, CS− and CS+ respectively, being such that the current supplies to each input, IN+ and IN− respectively, by said current source, CS− and CS+ respectively, eliminates current variations caused by voltage variations of the input voltage signal.

FIELD OF THE INVENTION

The present invention relates to a transconductance circuit intended toconvert a differential input voltage, supplied on two inputs, into adifferential output current. More particularly, the invention relates totransconductance intended to be implemented in an upconverter circuitand presenting a high linearity and low noise.

BACKGROUND OF THE INVENTION

Such a highly linear differential transconductance is presented inpatent U.S. Pat. No. 5,497,123. This differential transconductancecombines two sides each including a single input transconductance. Sucha transconductance is a class AB transconductance. This class isadvantageous as this is an intermediate between class A, where theconsumption is independent of the processed signal, and class B, wherethere is consumption only when a signal is processed. Advantages ofclass AB transconductance are also that they present a weak DC and arelinear on a large range. Such a transconductance is presented forimplementation in a reception chain. As such the transconductanceexhibits a low input impedance to match the image rejection filteroutput impedance. Consequently, the presented transconductance is welladapted to the reception chain as the obtained gain is inverselyproportional to the input impedance. A low input impedance is adapted todiscrete applications where the input of the transconductance is anoff-chip input.

For integrated applications the input impedance needs to be high inorder to reduce the consumption. Moreover, such a transconductance isnot adapted to the use in transmission chains where a large and linearinput impedance is generally asked for upconversion circuits.

SUMMARY OF THE INVENTION

The inventors have sought a design for a low-noise highly linear classAB transconductance presenting a high input impedance.

This aim and others are reached with a transconductance circuit aspresented in the introductory paragraph characterized in that, whereeach of the two signals of said differential input voltage is suppliedto each input through a follower transistor connected to said input byits emitter and receives said signal on a control electrode, each of thetwo inputs of the transconductance is connected to a respective currentsource that is dynamically controlled by the other input of thetransconductance, said current source being such that the currentsupplied to each input eliminates current variations caused by voltagevariations of the input voltage signal.

The invention combines the use of a common collector stage realizedthrough a follower transistor having its emitter connected to the inputand the use of a positive feedback from one input to the other in orderto cancel out current variations flowing in the follower transistors.Effectively, a simple follower transistor is not able to drive low inputimpedance with a large input voltage. The combination with a currentsource providing a positive feedback enables to cancel large variationsgenerated in the collector current of the follower transistor.Effectively, these current variations modulate the base-emitter voltageof said follower transistor and hence introduce distortion, unless it isdriven with a very large current. A linear, low-noise, high impedanceclass AB transconductance circuit is thus obtained.

An implementation of the invention is such that the transconductancecircuit comprises two sides, each side comprising an input, an output,at least a first transistor having a control electrode coupled forreceiving a bias voltage, a first electrode connected to said output anda second electrode connected to said input, a second transistor having afirst electrode and a control electrode coupled in common to said inputand a second electrode connected to a power supply terminal.

Advantageously, said first and second transistors are of the same size.

In an implementation of the invention, wherein each side furtherincludes a third transistor of the same size on said second transistor,said third transistor has a control electrode coupled to said firsttransistor and control electrodes of said second transistor, a firstelectrode connected to the output of the other side and a secondelectrode connected to said power supply terminal.

Such an implementation is known for providing a high linearity of thetransfer function of the transconductance for a large input voltage. Assaid third transistor is of the same size as said second transistor, thetransfer function is symmetrical with both negative and positive inputvoltages.

The first transistor handles a very large amount of current duringnegative excursion of the input voltage. In contrast, second and thirdtransistors handle a very large amount of current during positiveexcursion of the input voltage. Acting together, these transistorsprovide a transfer function which is linear to both positive andnegative input voltages whatever the relative size of said first andsecond transistors. In a simple implementation, the first and secondtransistors are of the same size.

An implementation of the current source is thus such that said currentsource includes a current mirror mirroring the current passing throughsaid second transistor with a gain of two. For instance, said currentmirror includes a mirror transistor of twice the size of said secondtransistor, said mirror transistor having a control electrode connectedto the first transistor and control electrodes of said secondtransistor, a first electrode connected to the input of the other sideand a second electrode connected to said power supply terminal.

The invention also relates to a chip intended to be implemented in atransmission chain and to a transmission device including such a chip.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a transconductance as described in theprior art;

FIG. 2 is a schematic diagram of a use of a transconductance accordingto the invention;

FIG. 3 a represents transfer functions of a transconductance accordingto the prior art and according to the invention;

FIG. 3 b represents the output currents for a transconductance of theinvention;

FIG. 4 represents a graph illustrating the performance of atransconductance according to the invention;

FIG. 5 schematically represents a chip according to the invention;

FIG. 6 represents a block diagram of a transceiver of radio frequencysignals according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 represents a transconductance circuit of the prior art. Thistransconductance circuit has two inputs IN+, IN− for receiving andifferential input signal, and outputs OUT+, OUT− for providing adifferential output signal. It includes two symmetrical sides eachcomprising three transistors T1+, T2+, T3+ for the first side and T1−,T2−, T3− for the second side. Said transistors are bipolar or MOStransistors having a collector, a base and an emitter respectively,corresponding to a first electrode, a control electrode and a secondelectrode.

Transistors T1+ and T1− have a control electrode coupled for receiving abias voltage, a first electrode connected to said output, OUT+ and OUT−respectively, and a second electrode connected to said input, IN+ andIN− respectively, biased by a constant current Ibias.

Transistors T2+ and T2− are in a diode configuration having a collectorand base coupled in common and an emitter coupled for receiving a powersupply voltage (e.g. ground) from a power supply terminal PST.

Transistors T3+ and T3− have a control electrode coupled to said firsttransistor and control electrodes of said second transistor, T2+ and T2−respectively, a first electrode connected to the output of the otherside, OUT− and OUT+ respectively, and a second electrode connected tosaid power supply terminal PST.

Connections of these transistors to the power supply terminal and to theinput or output of the circuit may be realized through resistors. Forexample, in FIG. 1, connection of second electrode of said firsttransistors T1+ and T1− to the input is realized through resistors R1+and R1−. Connections of said second and third transistors to the powersupply terminal PST are realized through resistors R2+, R2− and R3+,R3−. Said resistors have equal value in pairs and it is advantageousthat they all have the same value. Such a remark is also applicable tothe transconductance circuit of the invention represented in FIG. 2.

An input signal having a positive incursion in voltage relative to abias point Vbias applied, for example, to input IN+reduces the voltageacross the base emitter junction of transistor T1+. Conversely, avoltage across the base-emitter junction of transistor T2+ is increased.Transistor T3+ is a mirror transistor of a current mirror circuit formedby transistors T2+ and T3+. The increase in base-emitter voltage oftransistor T2+ is mirrored to transistor T3+, which increases a currentprovided at output OUT−.

Meanwhile, on the other side, an input signal having a negativeincursion in voltage relative to a bias point Vbias is provided to inputIN−. It has the opposite effect. A voltage across the base-emittervoltage of transistor T1− is increased by the negative voltage.Conversely, a voltage across the base-emitter junction of transistor T2−is decreased. The input signal having a negative voltage results in anincrease in current at output OUT− and a decrease in current at outputOUT+. The effects of the two sides of the transconductance go in thesame way.

This transconductance continues to perform linearly under extremeconditions where the signal current is equal or bigger than the biascurrent and is not limited by a current source that biases the circuit.On each side, either transistor T1 or transistor T2 is turned off due toa large input voltage. For example, an input signal having a largepositive voltage applied to input IN+ turns off transistor T1+, butincreases the current through transistor T2+ which is mirrored to theoutput OUT−. This output OUT− also receives the current from T1− whileT2− is turned off. Effectively, a large negative voltage applied toinput IN− turns off transistors T2− and T3−, but linearly increases thecurrent through transistor T1− which is provided at output OUT−. Thus,the transconductance remains linear even under large negative orpositive input voltage swings.

Nevertheless, as stated above, this transconductance has a low inputimpedance. An adaptation of the impedance could be done at each inputIN+ and IN− of said transconductance. The elements of such an adaptationof impedance, if active, present the drawback of adding distortion tothe input signal. Indeed, the current variations flowing in the active,and thus non linear adaptative circuit, generate distortion.

The effect of a signal δv of the input voltage is shown in FIG. 1. Sucha voltage variation +δv provided as IN+ results in a current variationδi in each of the two transistors T1+ and T1−. This current variation isthen mirrored and an amplified variation of 2δi is obtained on outputOUT+. A current I_(o)+2δI or I_(o)−2δi needs to be provided or absorbedby the impedance adaptation elements as the input is biased by aconstant current Ibias. The impedance adaptation elements willconsequently increase this variation and add distortion. For example,when the adaptation is realized through an emitter-follower transistor,the base-emitter voltage of said follower transistor should consequentlychange, leading to distortion. Moreover, it makes the noise contributionfrom the adaptative circuit high when this circuit is momentarily biasedat a low current.

FIG. 2 represents an exemplary use of a transconductance of theinvention that does not suffer from the above-presented drawbacks.

According to the invention, each of the two signals of said differentialinput voltage are supplied to each input IN+ and IN− respectively,through a follower transistor, TF+ and TF− respectively, connected tosaid input, IN+ and IN− respectively, by its emitter and receiving saidsignal on a control electrode. Moreover, each of the two inputs,respectively IN+ and IN−, of the transconductance is connected to acurrent source CS− and CS+ respectively, that is dynamically controlledby the other input of the transconductance IN− and IN+ respectively.Said current source, CS− and CS+ respectively, is such that the currentsupplied to each opposite input, IN+ and IN− respectively, by saidcurrent source, CS− and CS+ respectively, eliminates current variations2δi caused by voltage variations +δv and −δv of the input voltagesignal.

A positive feedback is thus implemented so as to provide the input witha current equal in magnitude to the one absorbed. The resulting inputimpedance Zin is then very high. In effect, a signal δv generates a weakcurrent signal δI=2δi−2δi, and the input impedance seen by the followertransistor Zin=δv/δI is very high. The follower transistor TF has nolonger current variations to be supplied to the input of thetransconductance and does not generate any distortion even at high inputvoltages.

In the following the functioning of the left side of FIG. 2 will bedisclosed. The description of the functioning of the right side issimilar. In FIG. 2, the case where resistors R1+, R2+ have identicalvalues and where transistors T1+, T2+ have the same size is illustrated.As seen previously, a voltage variation +δv generates a current that isdivided in, two in the branch including T1+ and in the branch includingT2+. Consequently, two current variations δi are generated in eachbranch. The current variation δi generated in T2+ is mirrored by T3+ tooutput OUT−. The effect of this current variation δi on output OUT− canbe added to the effect of the current variation generated in the branchincluding T1−. The same phenomenon is observed on OUT+ as the currentvariation δi generated through T2− is mirrored by T3− towards the outputOUT+.

FIG. 2 proposes an exemplary implementation of the current source, CS+and CS− respectively. Said current sources CS+ and CS− each include acurrent mirror including a mirror transistor, TM+ and TM− respectively,having a control electrode connected to the first and control electrodesof said second transistor, T2+ and T2− respectively, a first electrodeconnected to the input of the other side, IN− and IN+ respectively and asecond electrode connected to said power supply terminal PST.

The current is mirrored with a gain of two in order to cancel thecurrent variations entering at the opposite input. Transistors TM+ andTM− of a size that is double the size of T2 may thus be used. Inpractice, where transistors do not present an ideal behaviour, the sizeof said mirror transistor TM is adapted depending on the required rangeof output current. The connections of said mirror transistor TM+ and TM−to said power supply terminal may be realized through resistors RM+ andRM−. Such resistors are advantageously of the same value as the onesused with said second and third transistors. A transconductanceaccording to such a use presents a high input impedance symmetrical toboth positive and negative large voltage swings and a good linearity asillustrated in FIG. 3 a for a bias current of 5 mA. The prior arttransfer function illustrated by curve PAC is observed to be less linearthan the invention transfer function illustrated by curve IC. Thetransconductance of the invention is shown to be linear even for highoutput current. FIG. 3 b represent the two output currents I(OUT+) etI(OUT−) for the transconductance of the invention. Each output currentis not linear, but the differential output current I(OUT+)−I(OUT−)=Idiffis perfectly linear.

Indeed, each input of the transconductance, IN+ and IN− respectively, isdirectly biased by the feedback realized through the opposite currentsources, CS− and CS+ respectively, instead of being biased by a constantcurrent source.

The performance of the invention is illustrated in FIG. 4. This figureshows the results of a two-tone analysis performed on the implementationof the prior art, illustrated in FIG. 1 and on the implementation of theinvention presented in FIG. 2. The first tone is a differential voltagehaving an amplitude V and a frequency F1=375 MHz and the second tone isa differential voltage of the same amplitude V and a frequency F2=385MHz. These two tones are injected at the inputs of the transconductance.At the outputs of the transconductance, two main tones in current, IM1F1and IM1F2, are obtained at the respective frequencies F1 and F2. Twosecondary tones in current, IM3F1 and IM3F2, are also obtained atfrequencies F1′=365 MHz and F2′=395 MHz. These secondary tones resultfrom the non linearities of the transconductance. In FIG. 4 arerepresented the tone IM1F1 for the frequency F1 and the differencebetween the tones IM1F1 and IM3F1 for frequency F1. The higher thisdifference is, the more the transconductance linear is. Curves for F2would be similar. It is thus observed that, at identical bias current,for large input and output signals, the transconductance of theinvention is more linear than the one of the prior art. This isillustrated by the fact that the difference IM1F1−IM3F1 is larger andthat the slope of IM1F1 representing the gain of the transconductancedoes not fall for high input voltages. Moreover, the transconductance ofthe invention is observed to be higher than the one of the prior art.This is illustrated by the gain expressed by the IM1F1 curve. Forexample, it is observed that a transconductance of the invention cangenerate two differential tones having an amplitude of 15 mA with adifference IM1F1−IM3F1 of 40 dB and a global bias current of only 10 mA.The two differential tones are obtained with a gain variation below 1dB. The difference IM1F1−IM3F1 is 12 dB higher than the one obtainedwith the prior art at low signal levels and is even larger (up to 26 dB)at large input voltage.

FIG. 5 illustrates a chip FTCT intended to be implemented in atransceiver according to the invention. Said chip includes at least atransconductance TRCD as previously described and a mixer circuit MIXdedicated to provide a frequency shifted signal from the current outputfrom said transconductance TRCD.

FIG. 6 illustrates a block diagram of a transceiver FCS ofradio-frequency signals according to the invention. Generally, such atransceiver FCS is intended to receive and transmit signals through anantenna ANT. A commutation device COM controls the access to the antennaANT. Said commutation device COM is connected at least to a receptionchain RX and to a transmission chain TX. Said reception chain RXincludes at least a signal processing circuit SPC and a frequencytranslation unit FTCR, generally constituted by mixer circuits. Aprocessing unit MC follows these circuits. This processing unit MC alsoprocesses the signals to be transmitted and is thus connected to atransmission chain TX that includes at least a frequency translationunit FTCT implemented in a chip as previously described in FIG. 5 and anamplification unit AMPT. Such a transceiver is advantageously atelecommunication apparatus: mobile phone . . . . In this exemplaryapplication of the invention, the invention takes place in anupconverter circuit for which the characteristics of the invention areparticularly well adapted.

It is to be understood that the present invention is not limited to theaforementioned embodiments and variations and modifications may be madewithout departing from the spirit and scope of the invention as definedin the appended claims. In this respect, the following closing remarksare made.

It is to be understood that linearity of the transconductance can alsobe modulated conventionally through the values of the resistors of thetransconductance. Notably, modifications of the value of R_(o) enablesuch an improvement.

It is to be understood that isolation means can also be added to acircuit of the invention. For example, a transistor can be implementedin cascode with said third transistor, between the collector of saidthird transistor and the opposite output. This enables to avoiddisturbances caused by a charge of said transconductance, for example, amixer circuit.

It is to be understood that the invention is not limited to theaforementioned telecommunication application. The invention can be usedwithin any application using a reception chain needing a frequencytranslation before further processing. Radio-frequency application arethus involved in the invention.

It is to be understood that the present invention is not limited to theaforementioned mobile phone application. It can be used within anyapplication using a system where a drift frequency occurs, in automobileapplication for example.

It is to be understood that the method according to the presentinvention is not limited to the aforementioned implementation.

Any reference sign in the following claims should not be construed aslimiting the claim. It will be obvious that the use of the verb “tocomprise” and its conjugations does not exclude the presence of anyother steps or elements besides those defined in any claim. The article“a” or “an” preceding an element or step does not exclude the presenceof a plurality of such elements or steps.

1. A transconductance circuit to convert a differential input voltage, supplied as two signals on two inputs, into a differential output current, characterized in that, where each of the two signals of said differential input voltage is supplied to each input through a follower transistor connected to said input by its emitter and receives said signal on a control electrode, each of the two inputs of the transconductance circuit is connected to a respective current source that is dynamically controlled by the other input of the transconductance circuit, said current source being such that the current supplied to each input by said current source eliminates current variations caused by voltage variations of the input voltage signal, wherein the transconductance circuit comprises two sides, each side comprising an input, an output, at least a first transistor having a control electrode coupled for receiving a bias voltage, a first electrode connected to said output and a second electrode connected to said input through a resistor, a second transistor having a first electrode and a control electrode in common to said input and a second electrode connected to a power supply terminal, and wherein the first and second transistors of the first side are different from the first and second transistors of the second side.
 2. The transconductance circuit of claim 1, wherein said first and second transistors are of the same size.
 3. The transconductance circuit of claim 1, wherein each side further includes a third transistor of the same size as said second transistor, said third transistor has a control electrode coupled to said first transistor and control electrodes of said second transistor, a first electrode connected to the output of the other side and a second electrode connected to said power supply terminal.
 4. The transconductance circuit of claim 1, wherein said current source includes a current mirror mirroring the current passing through said second transistor with a gain of two.
 5. The transconductance circuit of claim 4, wherein said current mirror includes a mirror transistor of twice the size of said second transistor, said mirror transistor having a control electrode connected to the first and control electrodes of said second transistor, a first electrode connected to the input of the other side and a second electrode connected to said power supply terminal.
 6. A chip intended to be implemented in a transceiver comprising at least a transconductance as claimed in claim
 1. 7. A transceiver of radio-frequency signals comprising at least one chip as claimed in claim
 6. 